Dahlia bulbs, or tubers, are "tender"---that is, they'll die if left in the frozen ground over the winter. The good news is that it's easy to properly overwinter dahlia tubers for replanting the following spring. Timing is important because they do need a chill provided by the first frost of the season. Plan to dig dahlias immediately following that initial frost, when the foliage has browned and is dead or dying.
Cut the dahlia plant back to about 6 inches in height. Plunge a garden spade vertically into the soil about 9 to 12 inches from the dahlia stalk. Create a deep circle of similar cuts all the way around it. Shove the spade under the soil mass and lift it straight upward, using the handle as a lever because the soil will be heavy. Carefully remove as much excess soil as possible with your fingers without disturbing roots. Grasp the stalk and use it as a handle and gently work it back and forth to loosen the soil, then pull the plant straight up out of the soil mass.
Rinse the soil gently from the clump of tubers with the garden hose. Cut off the stalk and place the clump upside-down in a cool, dry location for several days. The tubers must air-dry completely to develop sturdy surface skin before they can be divided.
Examine the tubers to locate their 'eyes' or growing points. If a tuber isn't removed from the clump with an eye intact, it won't grow. The eye is a discernible round mark on the top of the tuber. Tubers usually grow from the large fleshy ridge joining them to the stem. Occasionally eyes will already have formed small shoots.
Use a sharp, clean knife to cut the individual tubers from the clump---don't just break them off. Make sure you cut into the stem to include a little piece of it with the tuber. This is important because the eyes will form sprouts at that point. Cut off any roots growing from the tubers.
Discard the mother plants tuber as well as any tubers that don't have at least one eye. Allow them to dry and cure for 24 hours in a cool, dry location.
Dust tubers with powdered fungicide if you wish. Fill a clear plastic bag half full of vermiculite and bury the tubers in it. Store this in a dark, cool place at about 35-45 F until you're ready to replant the tubers in the spring. Don't allow the tubers to freeze. A garage or basement works well for many gardeners; others choose their refrigerator's vegetable drawer.
Check the stored dahlia tubers once a month during the winter. Discard any that show signs of rot. If it appears that tubers are drying out and shriveling, add a few sprinkles of water to the surface of the vermiculite.